The globus pallidus is a structure commonly divided into two segments: the external segment (the globus pallidus in lower mammals is the homologus structure) and the internal segment (the entopeduncular nucleus is thought to be the homologous structure). It is likely that the ventral pallidum, a subcommissural extension of the external portion, should be considered a unique segment based on its limbic-related rather than motor-related afference, its neuropeptide content, and possibly limbic-related efference. Recently, this subcommissural compartment of the globus pallidus has been shown to encompass a larger region than previously thought, invading much of the basal forebrain area in both the rat and the monkey. It is set apart from the more dorsally placed pallidum both with respect to its circuitry and its histochemistry. The evidence available indicates that the afference to the ventral pallidum is derived solely from the limbic-related striatum and preliminary studies suggest that its efference may be a mixture, projecting to both motor-related and limbic-related structures. Furthermore, dense substance P-like immmunoreactivity is confined within the ventral pallidum while only weak staining is observed in the dorsal pallidum. Enkephalin-like imunoreactivity, on the other hand, is very dense in both pallidal components. The specific aim of this proposal is to study the afference and efference of the ventral pallidum, with special reference to the chemical specificity of its circuitry. While the motor-related circuitry of the dorsal pallidum has been studied extensively, little is known about the circuitry of the limbic-related ventral pallidum. It is likely that the limbic-afferented part of the pallidum not only has an efferent system which is different from the non-limbic part but that it continues this conduction line to non-motor related areas. Knowledge of these relationships may help identify brain structures involved in non-motor function of the striatum, conceivably including functions whose disorders can lead to mental illness.